1. Field of the Invention
This invention relates to adsorption filter units and associated processes for continuously removing a vaporous material from a gas.
2. Description of the Prior Art
There are many commercial processes in which organic solvents and the like are used in the processing or treating of materials. Typically, the cost of the solvent and other factors make it desirable to recover the solvent for repeated use. Moreover, ecological concerns make it desirable to prevent these solvents from being exhausted into the air. Much of the solvent may become vaporized during use, and recovery methods therefore must include means for recovering this vaporized solvent.
Recovery of the volatized organic solvent or the like is generally accomplished by passing the gas, typically air, in which the solvent is supported through an adsorption filter. The filter contains a solid adsorbent, such as activated carbon or charcoal, which adsorbs the vaporous solvent from the supportive gas. The solvent is then removed from the charcoal and collected, with the filter also being regenerated by this procedure for further use.
Various methods and devices have been utilized for solvent recovery by adsorption. In U.S. Pat. No. 3,772,854, issued to Tamura et al. on Nov. 20, 1973, there is disclosed a process for removing sulfur oxides from exhaust gases. The sulfur oxides are adsorbed by charcoal filters and are then removed therefrom by immersing the filters in water to cause formation of sulfuric acid which is retained in the water. The dilute sulfuric acid thereby obtained is collected and concentrated by continuous vaporization of the water present therein. Unlike the present invention, the Tamura et al. procedure removes the adsorbed material from the carbon filters by dissolving the adsorbed sulfur oxides in water. There is no suggestion that the water be heated to vaporize the sulfur oxides from the filter, and indeed this in itself would only result in an exhaust gas containing sulfur oxides, much like the original feed to the adsorption unit. Necessarily, immersion of the carbon filter of Tamura et al. in water causes dissolution of the sulfur oxides in the form of sulfuric acid, but the acid solution is collected in the liquid state and no suggestion of methods or advantages in vaporizing the acid solution is provided. Moreover, it would be the normal expectation that vaporization would be undesirable since this would increase energy requirements, handling and processing equipment needs, and safety precautions.
A method for collecting ammoniacal gases and revivifying animal black filters is disclosed in British Pat. No. 3275, issued to Brooman on Dec. 31, 1861. In accordance with the Brooman procedure, the animal black is restored in its capacity to absorb lime by pouring a boiling solution of carbonate of soda, or caustic soda, onto the animal black in the filter. The solution remains in contact with the animal black for about one hour, with the energy being increased by the injection of steam if needed. The solution is then drained and steam is injected to remove all traces of ammoniacal matters, which have been formed by the action of the carbonate of soda or caustic soda upon the saccharine liquors previously absorbed by the animal black.
In contrast to the present invention, Brooman does not suggest that the soda solution be raised to and maintained at its boiling point, or more precisely at the vaporization temperature of an azeotrope of the solution and the material to be removed from the filter material. Instead, the soda solution is utilized to react with the saccharine liquors to form ammoniacal matters which are driven off by the injection of steam after the soda solution has been removed. In the Austrian Pat. No. 121,736, an apparatus and method are disclosed which relate to a technique for cleaning dust filters. In this type of application, immersion of the filter in a liquid bath is employed to physically disrupt the filtered material. Similarly, in the process of the Brooman patent, the filter is immersed in a liquid bath in order to physically remove the adsorbed materials by chemical reaction and dissolution.
In U.S. Pat. No. 3,089,250, issued to Victor on May 14, 1963, there is disclosed a method for recovering an adsorbed solvent from a charcoal filter. In the Victor method, the adsorbent charcoal is supported upon a perforate surface within a container. Steam is then admitted into the container and contacts the adsorbent to strip the solvent therefrom. A surface located within the container and below the perforate surface is also heated to a temperature above the boiling point of water, and acts to revolatize any condensate which is formed by the steaming operation.
A similar solvent recovery method is disclosed in U.S. Pat. No. 3,332,854, issued to Duckstein on July 25, 1967. The Duckstein patent discloses the use of activated charcoal filters, either singly or alternately, for the recovery of a solvent from a gas stream. When used alternately, the vaporous solvent is adsorbed by one or more filters, while the remaining filters are subjected to a steam desorption process. The Duckstein apparatus passes the solvent laden air vertically through a charcoal bed supported on a sieve. Regeneration and recovery is accomplished by passing superheated steam up through the sieve and into a condensor and separator.
A somewhat different function is performed by the regeneration system disclosed in U.S. Pat. No. 3,362,888 issued to Ricigliano on Jan. 9, 1968. The solvent is adsorbed by a filter in the usual way. An amount of the pure solvent is then vaporized and is passed through the spent filters. As the vaporized solvent passes upwardly through the filters, dirt and sludge is forced off of the filters. Removal of the sludge revivifies the filters, and also permits recovery of the solvent contained within the filters.
Various apparatus for use in filter restoration are also known in the art. U.S. Pat. No. 239,962, issued to Jennings on Apr. 12, 1881, discloses a procedure for regenerating filters by forcing superheated steam through the charcoal in the filter. A pair of filters are employed to enable use of one while the other is being regenerated. In U.S. Pat. Nos. 1,842,316, issued to Coulter on Jan. 19, 1932; 1,868,581, issued to Miller on July 26, 1932; and 2,087,157, issued to Lind on July 13, 1937 are disclosed less related regeneration systems.
In general, the prior art methods utilize either a steam or vapor stripping process in which intimate contact between the adsorbed material and the desorbing steam or vapor is not available, or immersion of the filter in a liquid with the solvent being removed from the filter by entering into solution or reacting with the liquid. In order to obtain efficient operation of these processes either complex and expensive structures or the use of superheated steam is required. It would therefore constitute a significant improvement over the prior art techniques if the adsorbed solvent could be recovered and the filters regenerated in a simpler and less expensive manner. It is also desirable that the regeneration of the filters be accomplished in a short amount of time in order to minimize the period during which a given filter is undergoing desorption and therefore is not in use.